In the past, there has been known an EGR (Exhaust Gas Recirculation) system which serves to introduce an exhaust gas of an internal combustion engine into an intake system of the internal combustion engine as an EGR gas. By supplying the EGR gas to the internal combustion engine, it is possible to attain a decrease of NOx in the exhaust gas and an improvement in fuel economy.
In addition, in recent years, there have been developed internal combustion engines which are each provided with a low pressure EGR device and a high pressure EGR device. The low pressure EGR device has a low pressure EGR passage which is connected at one end thereof to a portion of an exhaust system at the downstream side of a turbine of a supercharger, and is also connected at the other end thereof to a portion of an intake system at the upstream side of a compressor of the supercharger. An EGR gas is introduced into the intake system through the low pressure EGR passage. The high pressure EGR device has a high pressure EGR passage which is connected at one end thereof to a portion of the exhaust system at the upstream side of the turbine of the supercharger, and is also connected at the other end thereof to a portion of the intake system at the downstream side of the compressor of the supercharger. An EGR gas is introduced into the intake system through the high pressure EGR passage. A low pressure EGR valve is arranged in the low pressure EGR passage, and a high pressure EGR valve is arranged in the high pressure EGR passage. The flow rates of the EGR gases in the individual EGR passages are controlled by the individual EGR valves, respectively.
In addition, an NOx selective reduction catalyst may be arranged in the exhaust system of the internal combustion engine as an exhaust gas purification catalyst. In this case, an ammonia derived compound, which is a reducing agent, is added from an ammonia derived compound addition device which is arranged in the exhaust system at the upstream side of the NOx selective reduction catalyst, so that the ammonia derived compound is supplied to the NOx selective reduction catalyst.
In a patent document 1, there is disclosed a construction in which a supply control valve for supplying an aqueous urea solution and an NOx selective reduction catalyst are arranged in an exhaust passage at the downstream side of a connection portion of a low pressure EGR passage. In such a construction, when the aqueous urea solution is supplied from the supply control valve, the pressure in the low pressure EGR passage in the surrounding of an inflow end thereof will go up to a large extent due to the evaporation of the aqueous urea solution. As a result, there will be a fear that the amount of low pressure EGR gas may increase more than a target amount. Accordingly, in the patent document 1, in order to maintain the amount of low pressure EGR gas at the target amount, the degree of opening of the low pressure EGR valve is corrected in a decreasing direction when the aqueous urea solution is supplied from the supply control valve, in comparison with the case when the aqueous urea solution is not supplied.